This invention relates to corona generating devices for applying electrostatic charge onto a suitable surface. More particularly, this invention relates to a novel corona generating device for use in electrostatography, in which the voltage applied to the corona generating devices is relatively high.
In the electrostatographic process, an electrostatographic plate comprising a photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity which reaches them and thereby creates an electrostatic latent image on or in the plate coating which may then be developed into a visible form by applying a developer material to the plate using any one of a number of development techniques generally known and used in the art. The developer material electrostatically clings to the plate in a visual pattern corresponding to the electrostatic image. Thereafter, the developed image is usually transferred from the plate to a support material, such as paper, to which it may be fixed by any suitable means thereby forming a permanent print.
The charging of the electrostatographic plate in preparation for the exposure step is accomplished by means of a corona generating device whereby an electrostatic charge is applied to the electrostatographic plate to raise it to a positive or negative potential in the range of approximately 600 to 1,000 volts. Examples of corona generating devices for this purpose are disclosed in U.S. Pat. Nos. 2,777,957 and 2,836,725. In U.S. Pat. No. 2,777,957, a plurality of parallel wires are connected to a high voltage source and supported in a conductive shield that is arranged in closely spaced relation to the surface to be charged. When the wires are energized with suitable high voltage, corona is generated along the surface of the wires and ions are caused to be deposited on the adjacent photoconductive surface. Suitable means are usually provided to effect relative movement between the surface to be charged and the corona generating device. Such a device may alternatively have a single corona wire, as shown in U.S. Pat. No. 2,836,725.
As indicated in U.S. Pat. No. 4,110,614, the conventional form of corona discharging device for use in reproduction systems generally involves the connection of a conductive corona electrode in the form of an elongated wire to a corona generating high voltage source. As indicated in that patent, several problems have been found with such corona devices. These problems include inability to deposit a relatively uniform charge, the growth of chemical compounds on the coronode which eventually degrades the operation of the corona device, and the degradation in charging output resulting from contamination of the coronode by toner particles, etc. In said U.S. Pat. No. 4,110,614, there is disclosed a corona charging device which employs a corona discharge electrode which comprises a wire coated with a relatively thick layer of dielectric material, such as a glass coating. The generation of ions is accomplished by means of an AC electric field established at the dielectric surface by capacitive coupling through the dielectric material. The flow of ions to the surface to be charged is regulated by means of a bias applied to a conductive biasing member which establishes an electric field between the surface to be charged and the member. Such a dielectric coated corona device generally requires the use of higher coronode voltages for the generation of charge, for example, 8,000 to 10,000 volts peak or higher.
The use of such higher voltages, together with the presence of a conductive biasing member, have caused problems in the degradation of the endblocks or the mounting means for the corona discharge electrode or wire. This degradation may be appreciated from the fact that the corona discharge electrode is at a high electric potential and there is a large difference in potential between the corona discharge electrode and the surrounding structures. The high voltage on the corona discharge electrode can lead to streaming or arcing and a corona around the electrode in the endblock region. Streaming is sometimes seen as a glow-like discharge. Arcing may be seen as sparks and the corona region is the result of partial ionization of the air surrounding the electrode. Electric arcs will, of course, attack the materials with which the arcs are in contact. The corona region contains ions or charged particles, and such charged particles will attack the endblocks or other structures which support or mount the electrode. The large potential difference between the corona discharge electrode and the biasing member, and between the corona discharge electrode and the mounting means for the electrode, may cause electrical discharges within the endblocks. Although the precise scientific explanation for such electrical discharges are not known, it is believed that minute voids in the endblock materials permit the electrical discharges and corona to take place within the voids. The discharges carbonize and enlarge the voids, which leads to bigger discharges, which leads to bigger voids, ultimately resulting in the complete failure of the endblocks. This process is sometimes referred to as a channeling mechanism.
Other factors contributing to the erosion of the endblock structure include the presence of ozone and other chemicals in the atmosphere near a corona generating device.
Prior art endblocks were generally constructed of highly dielectric plastic materials such as nylon or polyvinylchloride. See, e.g., U.S. Pat. Nos. 4,110,811 and 4,112,298. Although such plastic materials are strongly dielectric, they cannot prevent or withstand arcing or sparking upon prolonged exposure to high voltages, particularly such high voltages used with a dielectrically coated coronode. Accordingly, there is a need for improved endblocks or mounting means for corona electrodes.